Add the water and starter to a large mixing bowl and whisk to combine. Add the flour and salt and stir with a wooden spoon or mix with your hands to form a shaggy dough. The dough will feel a bit stiff but will mix together.
300 grams water, 100 grams active 100% hydration sourdough starter, 500 grams whole grain einkorn flour, 10 grams sea salt
Set the dough aside to rest for an hour, covered with a tea towel and large plate.
After an hour, you can begin stretches and folds. Complete one round of stretching and folding, then let the dough rest for 30 minutes before doing the next one. Do four total rounds of stretches and folds over the course of two hours.
Line a proving basket with a clean tea towel and sprinkle with a bit of flour. Set aside.
Turn the dough out onto a clean work surface and lightly flour the top of the dough (do not flour the countertop). Rotate the dough as you usually would to form a boule with surface tension. Be careful at this point as the whole grain present can cause tearing in the surface 'skin' quite easily.
Place the dough seam-side up into the prepared basket. Cover well and let it rise at room temperature for 2-3 hours, depending on the temperature, or until it's increased in size by about ⅓.
Keep the loaf covered and refrigerate it overnight. It should rise more during that time to approximately double in size in total.
In the morning, preheat a large Dutch oven or other heavy lidded pot in the oven at 230°C (450°F) for about half an hour.
Once the oven is hot, gently turn the dough out onto a sheet of parchment paper. Remove the pot from the oven and carefully place the loaf into the pot.
Bake with the lid on for 20 minutes.
Remove the lid and bake for another 20-25 minutes, or until the loaf is as dark as you'd like it to be. It will be cooked through at the 40 minute mark.
Remove from the oven and cool the loaf fully before slicing. If possible, you can take the loaf out of the pot to cool on a wire rack before slicing (if it's too difficult to do without burning yourself, simply let it cool in the pot). It will be gummy if sliced too soon.
Store the bread in a paper or cloth bag at room temperature for several days, or freeze if preferred.